If you're playing someone competent, it'll never work. The entire cast can shield before Up-B comes out or has a way to hit you after jab, either before they land (anyone with an aerial frame 5 or faster) or after (high gravity characters with a frame 2 jab).
It's more useful to do a D-Smash/Tilt, grab, or shield mixup. That said, Jab > anything fails to work on a fair number of the cast because Luigi can't even shield after Jab 2 and be safe. It's finish the Jab combo or bust. I say Jab 2 because it lets you act 1 frame faster than Jab 1.
The only reason it would work is:
a) opponent doesn't know the matchup
b) opponent accidently air-dodged while trying to shield on landing/buffered shield/buffered an aerial that isn't fast enough to beat it AND has enough landing lag to make the hit possible.